Method and Apparatus for Sensing of Levitated Rotor Position

ABSTRACT

A pump with magnetically-levitated rotor includes a position sensor having an eddy-current sensor coil that operates as a resonating element in a low frequency oscillator located within the pump housing. The oscillator is operably interconnected with additional electronics that shift the frequency of the oscillator output signal to a lower frequency. The lower frequency signal is directed to a frequency measurement circuit that provides a value representing a position of the rotor.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/613,307, filed Mar. 20, 2012, entitled “Method and Apparatus forSensing of Levitated Rotor Position,” which is incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates generally to position sensors, and morespecifically relates to position sensors for magnetically-levitatingpumps, such as cardiac assist pumps that may be implanted in a patient.

BACKGROUND OF THE INVENTION

Rotor dynamic pumps, such as centrifugal, mixed-flow, and axial-flowpumps with mechanical bearings or magnetically suspended systems, havebeen widely used as a ventricular assist device to support patients withheart diseases. In magnetically-levitated blood pumps, which generallyinclude an impeller or rotor that is both magnetically suspended androtated without mechanical means, the magnetic bearings may be used toconstrain motion in a longitudinal direction and active elements may beused to control a lateral position of the rotor. There is a relativelynarrow region of travel along the longitudinal axis over which thisconstraint applied by the magnetic bearings occurs, and over which thereis an adequate force to maintain the concentricity of the rotatingelement (e.g., rotor) with the longitudinal axis. The magnetic bearingforces tend to push the rotor longitudinally away from this narrowfunctional region. A control system is used to sense the longitudinalposition of the rotor, and based on this position, apply a force tocounter the travel away from the intended location and maintain therotor in the desired longitudinal position.

For magnetically-levitated pumps, it has historically been a difficultproblem to determine the rotor's longitudinal position with suitableprecision and with a sufficient bandwidth to maintain a stable positionalong the longitudinal axis. This difficulty is due, at least in part,to the sealed nature of the fluid flow path through the stator. Inparticular, in blood pumps, titanium alloys are used for compatibilitywith the blood. The use of titanium requires that the rotor positionsensor be able to sense the position of the rotor through at least onelayer of titanium.

Existing control systems that attempt to address these problems relatedto magnetically-levitated pumps have suffered from a number ofshortcomings. For example, such systems are sensitive to externalelectrical noise such as radio signals, changing magnetic fields withinthe pump, temperature changes in the pump, temperature changes in thecontroller, and temperature changes in the cable that connects the pumpto the controller. These systems also have limitations related tominimizing a size of the pump housing because of the large number andsize of the electronics that are typically required to be positionedinside the pump housing. The sensitivity to changes in cable impedanceis also problematic due to fluid ingress, flexure or other reasons.

SUMMARY OF THE INVENTION

Various embodiments of position sensors for magnetically-levitated pumpsare set forth herein in accordance with the present disclosure.

In accordance with one embodiment of the present disclosure, amagnetically-levitated pump includes a position sensor having aneddy-current sensor coil that operates as a resonating element in a lowfrequency oscillator located within the pump housing. The oscillator isoperably interconnected with additional electronics that shift thefrequency of the oscillator output signal to a lower frequency. Thelower frequency signal is directed to a frequency measurement circuitthat provides a value representing a position of the rotor along thelongitudinal axis of the pump. The value may be in the form of, forexample, a binary number, an electrical circuit, an electrical voltage,or other representation.

An alternative position sensor includes an eddy-current sensor coil thatoperates as a resonating element in a low frequency oscillator that ispositioned within the pump housing. An output from the oscillator may beoperably connected with the input to a phase locked loop or frequencylocked loop. The output of the phase locked loop or frequency lockedloop may be a feedback value that represents a position of the rotoralong the longitudinal axis of the pump. The feedback value may be inthe form of, for example, a binary number, an electrical current, anelectrical voltage, or other representation.

Either of the example position sensors described above may be associatedwith at least one memory element that stores calibration,characterization, correction, and other parameters for the pump or thecontroller. The parameters may permit the pump to be operablyinterchanged with any similar pump. For example, a common controllerpositioned remotely of the pump housing may be used with any of a numberof different pumps because each pump separately carries a number of theparameters stored in memory that are accessible by the controller.Similarly, any given pump may be used with a plurality of differentcontrollers because of parameters of the controller that are stored bythe controller.

The frequency measurements in the example circuitry described herein mayinclude a high speed counter that is configured to measure an intervalof time between cycles of the low frequency signal. The number of cyclesof the low frequency signal may be measured over a specified timeinterval. A microprocessor of the pump system may include such atimer/counter subsystem.

Another aspect of the present disclosure relates to a pump systemconfigured to provide fluid flow and that includes a stator housing, arotor hub, and an eddy current sensor coil. The stator housing has aninlet, an outlet and a fluid pathway. The rotor hub is disposed withinthe fluid pathway between the inlet and the outlet, and includes a bodyhaving a leading portion positioned adjacent the inlet and a trailingportion positioned adjacent the outlet. The eddy current sensor coil ispositioned external the fluid pathway and operable to determine aposition of the rotor hub relative to the stator housing. The sensorcoil operates as a resonating element in a low-frequency oscillator.

The pump system may include a frequency shifting device that shifts afrequency of an output signal from the oscillator to a lower frequencysignal. The pump system may include a frequency measurement circuit thatmeasures the frequency of the lower frequency signal and outputs a valuerepresentative of a position of the rotor hub relative to the statorhousing. The value that is output from the frequency measurement circuitmay be in the form of a binary number, an electrical current, or anelectrical voltage. The pump system may include a phase-locked loop thatlocks a phase of an output from the oscillator. The pump system mayinclude a frequency-locked loop that locks a frequency of an output fromthe oscillator. The pump system may include a memory element containingat least one of calibration, characterization, and correction parametersfor the sensor coil.

The pump system may include a high speed counter configured to measurean interval of time between cycles of the lower frequency signal. Thepump system may include a high speed counter configured to measure anumber of cycles of the lower frequency signal over a specified timeinterval. The pump system may include a pump housing, wherein the statorhousing, rotor hub, and sensor coil are positioned within the pumphousing. The pump system may include a microprocessor positioned remotefrom the pump housing. The pump system may include a permanent magnetbearing and a magnet motor, wherein the magnet motor includes a motormagnet carried by the rotor hub and a motor coil carried by the statorhousing. The at least one permanent magnet bearing levitates the rotorhub within the stator housing, and the magnet motor is operable torotate the rotor hub within the stator housing.

Another aspect of the present disclosure relates to a sensor assemblyfor a pump with a magnetically-levitating rotor. The sensor assemblyincludes a sensor coil positioned on a stator of the pump, a rotor ofthe pump that is arranged within the stator and having a conductivesurface, and a low frequency oscillator positioned within a housing ofthe pump. The sensor coil may operate as a resonating element in the lowfrequency oscillator in response to a change in relative positionbetween the conductive surface and sensor coil. An output signal fromthe low frequency oscillator is used to adjust a position of the rotorrelative to the stator. An output of the low frequency oscillator may bein the range of about 200 kHz to about 350 kHz. An output of the lowfrequency oscillator may be a sine wave signal.

Another aspect of the present disclosure relates to an activemagnetically levitating pump system configured to provide fluid flow.The pump system includes a stator housing having a fluid pathway, arotor disposed within the fluid pathway, and an eddy current sensorcoil. The eddy current sensor coil is positioned external the fluidpathway and operable to determine a position of the rotor with respectto a defined axis of the stator housing. The sensor coil operates as aresonating element in a low-frequency oscillator.

The eddy current sensor coil may be operable to determine a position ofthe rotor with respect to a longitudinal axis of the stator housing. Theeddy current sensor coil may be operable to determine a position of therotor with respect to a lateral axis of the stator housing. The pumpsystem may include a pump housing, wherein the stator housing, rotor andeddy current sensor coil are positioned in the pump housing. Thelow-frequency oscillator may be positioned in the pump housing.

A further aspect of the present disclosure relates to a method ofdetermining a rotor position in a stator housing. The method includesproviding a stator housing having a fluid pathway, a rotor hubpositioned in the fluid pathway, a sensor coil positioned external thefluid pathway, and an oscillator. The method further includes inducingeddy currents in the rotor hub via the magnetic field of the sensorcoil, which eddy currents in turn produce magnetic fields that interactwith the magnetic fields of the sensor coil as the rotor hub is movedrelative to the stator housing, wherein the sensor coil operates as aresonating element in the oscillator. The method also includesdetermining a position of the rotor hub relative to the stator housingusing an output of the oscillator.

The method may include shifting a frequency of a signal that is outputfrom the oscillator to create a lower frequency signal, measuring thelower frequency signal, and providing a value for the measured lowerfrequency signal that represents a position of the rotor hub relative tothe stator housing. The method may include locking a phase of a signaloutput from the oscillator to create a phase locked signal, measuringthe phase locked signal, and providing a value for the measured phaselocked signal that represents a position of the rotor hub relative tothe stator housing. The method may include locking a frequency of asignal output from the oscillator to create a frequency locked signal,measuring the frequency locked signal, and providing a value for themeasured frequency locked signal that represents a position of the rotorhub relative to the stator housing. The method may includemagnetically-levitating the rotor hub in the fluid pathway, andcontrolling a longitudinal position of the rotor hub relative to thestator housing in response to the determined position of the rotor hub.

Another example method in accordance with the present disclosure relatesto determining a position of a rotor within a housing in amagnetically-levitating pump system. The method includes providing acontroller and a pump, wherein the pump includes a rotor, a stator, aposition sensor, and an oscillator, the rotor is positioned in thestator, the position sensor is positioned outside of the stator. Theposition sensor includes a coil. The method includes creating a changein frequency in the oscillator with the position sensor in response to achange in position of the rotor relative to the stator, processing anoutput signal of the oscillator with the controller to create acorrection or error value representative of the change in relative axialposition, and correcting a position of the rotor based on the value.

The method may also include correcting the position of the rotor with avoice coil. The method may include providing a controller that ispositioned remote from the oscillator and includes a microprocessorconfigured to determine the correction or error value using the outputsignal of the oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a block diagram showing an example pump rotor levitationsystem in accordance with the present disclosure.

FIG. 2 is a block diagram showing additional features of the pump systemof FIG. 1.

FIG. 3 is a circuit diagram showing circuit components of the pumpsystem of FIG. 1.

FIG. 4 is a mechanical diagram showing bearing and positioningcomponents of a pump housing of the pump system of FIG. 2.

FIG. 5 is a flow diagram showing an example method in accordance withthe present disclosure.

FIG. 6 is a flow diagram showing another example method in accordancewith the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments are described more fully below in sufficient detail toenable those skilled in the art to practice the system and method.However, embodiments may be implemented in many different forms and thepresent disclosure should not be construed as being limited to theembodiments set forth herein. The following detailed description is,therefore, not to be taken to be limiting in any sense. For purpose ofillustration, discussions of the technology will be made in reference toits utility as a cardiac assist blood pump. However, it is to beunderstood that the technology may have a variety of wide applicationsto many types of turbomachinery including, for example, commercial andindustrial pumps, compressors, and turbines.

The present disclosure is directed to a magnetically-levitated pumpsystem that includes a pump and a controller. The pump and controllerare typically positioned remote from each other and interconnected witha cable. The pump includes a magnetically-levitated pump and somecircuitry. The controller includes a microprocessor and other circuitry.The pump includes a position sensor that senses a position of a rotorthat is arranged within a stator of the pump. The position sensordetermines a position of the rotor in a longitudinal direction withrespect to an inlet or outlet of a flow path through the stator.

The positioning sensor (also referred to as a resonance sensor) mayinclude a coil that acts as a frequency determining element of a radiofrequency oscillator positioned within a housing of the pump. The outputof the oscillator is directed to a frequency measuring mechanism, theoutput of which is directed to an analog or digital converter to obtaina sensor output. Described in another way, the sensor coil acts as aresonant member of an oscillator that varies in accordance with relativeaxial movement between the rotor and stator. The oscillator, alsopositioned within the pump housing in proximity to the pump, provides anoutput signal that is output at a relatively high level (e.g., at about1 volt peak to peak) through the cable to the controller where thecontroller operates to measure that frequency. The measured frequency iscorrelated to an axial position of the rotor relative to the stator. Theoutput from the controller may be used as a position value then used bya position altering device of the pump (e.g., a voice coil) to move therotor relative to the stator.

The circuit components positioned inside the pump housing may beselected such that the circuitry of the pump system is less subject toexternal electromagnetic fields and radio signals. The circuitryreplaces the quadrature sensor and related Wheatstone bridge used forthe coil position sensor in prior devices. In one example, the circuitryutilizes a Colpitts oscillator or other oscillators well known in theart that have been updated to use transistors and require a relativelysmall number and size for the surface mount components.

Referring now to FIG. 1, an example magnetically-levitated pump system10 is shown schematically including a position sensor coil 25 (alsoreferred to as a rotor position sensor or a rotor position sensor coil),an oscillator 28, a controller 14, and a rotor positioning device 24(also referred to as a voice coil 24). The position sensor coil 25,oscillator 28 and rotor positioning device 24 are typically positionedwithin a housing of the pump. The controller 14 is typically positionedremote from the pump. The coil position sensor 25 is typically mountedto an exterior of a stator of the pump and includes a wire coil. A rotorpositioned within the stator may carry or comprise a conductivematerial. The rapidly changing magnetic field induced by oscillatorcurrent flowing through the position sensor coil 25 causes currents tobe induced in a conductive surface of the rotor. These currents, inturn, produce a magnetic field which interacts with thesense-coil-induced magnetic field, and affect a flow of energy from acurrent in the position sensor coil 25, to a magnetic field, and back toa current in the position sensor coil 25. This change is reflected as achange in inductance of the coil position sensor 25, which is ameasurable parameter. The change in inductance, in turn, changes thefrequency of the oscillator 28.

An output of the oscillator 28 is delivered to the controller 14 andconverted to a value that represents a relative position of the rotor tothe stator. This value may be communicated to a rotor positioning device24 that adjusts the position of the rotor relative to the stator. Therotor positioning device 24 may be a voice coil. The voice coil maycomprise a plurality of wire coils positioned on an exterior of thestator and a magnet carried by the rotor that is aligned radially withthe wire coils of the voice coil.

U.S. Published Patent Application No. 2011/0237863 discloses variouscomponents of a magnetically-levitated pump such as the rotor, stator,magnetic bearings and voice coil, and is incorporated herein in itsentirety by this reference. Many other types of pumps and motors (e.g.,a reciprocating pump) having different configurations that utilizevarious technologies may also be applicable to the present disclosure.FIG. 4 shows schematically an example magnetically-levitated pump 112having at least some of the same or similar components included in pump12. Pump 112 includes levitation components 122, rotor positioncomponents 125, a stator housing 130, a rotor 132, fixed bearing magnets134, 136 mounted to the stator housing 130, and suspended bearingmagnets 140, 142 carried by the rotor 132. The levitation components 122include a voice coil 138 mounted to the stator housing 130, and firstand second voice coil magnets 144,146 carried by the rotor 132. Therotor position sensor 125 includes a sensor coil 126 mounted to thestator housing 130. The stator housing 130 includes a fluid channel 150,an inlet 152 and an outlet 154. The rotor 132 includes first and secondends 156, 158. The first end 156 may function as a conductive sensortarget. The rotor position sensor 125 may operate to determine aposition of the rotor 132 with respect to an axis or other feature ofthe stator housing 130, such as a longitudinal position or a lateralposition with respect to a longitudinal or lateral axis of the statorhousing 130, respectively.

FIG. 2 illustrates additional components of the pump system 10. The pumpsystem 10 includes a pump 12, the controller 14, and a patient/clinicalinterface 15. The pump 12 and controller 14 are typically interconnectedwith a cable that provides electronic communication between the pump 12and controller 14. The patient/clinical interface 15 may be local toeither the pump 12 or controller 14 (e.g., included in the housing ofthe controller 14) or positioned remotely via an electronic connection.The electronic connection between any of the pump 12, controller 14, andpatient/clinical interface 15 may be wired or wireless, wherein thewired or wireless connection may be accomplished at least in part usinga communications network such as the Internet.

The pump 12 may include, in addition to the stator housing and the rotorpositioned within the stator housing, a rotor position sensor 25, anoscillator 28, a sensor coil 26, a levitation drive 22, a voice coil 24,and a motor drive 20 that includes a plurality of pump coils 21. Therotor position sensor 25, levitation drive 22, and motor drive 20 mayeach include at least one magnet that is carried by the rotor andpositioned within the stator. The sensor coil 26, voice coil 24, andpump coils 21 are typically all positioned outside of the stator or atleast outside of a flow pathway within which the rotor is positioned.

The controller 14 comprises one or more microprocessors 36, a motorcontrol 40, the rotor position sensor 25, a levitation control 42, acontroller monitoring device 46, a data logging device 48, a battery andcharger 50, a power supply 52, a power management device 54, an externalcommunication 56, and a levitation system 58 (e.g., a PID, VZP and PWM).The microprocessors 36 communicate with each of the rotor positionsensor 25, levitation drive 22 and motor drive 20. The rotor positionsensor 25, levitation drive 22 and motor drive 20 may communicate withthe oscillator 28, voice coil 24, and pump coils 21, respectively, viaelectronic communication provided by a cable interconnecting the pump 12and controller 14.

The patient/clinical interface 15 may include a clinical user interface62, a patient user interface 64, and a wall power interface and charger66. The clinical user interface 62 may be provided locally or remotelyrelative to the controller 14 and pump 12. Similarly, the patient userinterface 64 may be carried by the controller 14 or the pump 12. Theclinical user interface 62 and patient user interface 64 may provide aclinician and patient with the ability to control, modify, and receivefeedback from the pump 12 and controller 14.

Referring now to FIG. 3, various circuit components of the pump system10 are shown. The pump 12 may include the sensor coil 26 connectedelectronically to the oscillator 28. As described above, the sensor coil26 may act as a resonating element for the oscillator 28. The pump mayalso include a stator 16 (also referred to as a stator housing) androtor 18 (also referred to as a rotor hub). The sensor coil 26 may bemounted to the stator 16 and positioned external of a flow pathway ofthe stator within which the rotor 18 is positioned.

An output of the oscillator 28 may be directed to a multiplier 30 (alsoreferred to as a mixer 30) of the controller 14. Controller 14 alsoincludes a low pass filter 32, a comparator 34, at least onemicroprocessor 36, and an amplifier 38. The microprocessor 36 may be anMSP430 processor. The microprocessor 36 may provide a local oscillatorsignal 37 back to the multiplier 30. A crystal 39 may be associated withthe microprocessor 36 to provide electronic stability. Themicroprocessor 36 may include a timer/comparator input to receivesignals from the comparator 34, and a DAC output connected to theamplifier 38. The amplifier 38 may provide a sensor output 43 that isdirected back to levitation circuitry of pump 12 (e.g., a voice coilrotor positioning device) to adjust a position of the rotor 18 relativeto the stator 16. Alternatively, the sensor output 43 may be coupled tothe levitation circuitry in a digital fashion, without need of a DAC oramplifier.

In one example, the oscillator 28 typically oscillates at around 250kHz, and preferably somewhere in the range of about 220 kHz to about 350kHz. The multiplier 30 may be a diode double balanced mixer. Themultiplier 30 may mix the signal received from the oscillator 28 withthe local oscillator signal 37 received from the microprocessor 36. Inone example, the microprocessor 36 may be running at about 20 mHz, whichfrequency may be divided down to whatever frequency is required for thelocal oscillator signal 37 for use in calibrating.

A frequency range received in the signal from pump 12 is measured, andfrom that measured frequency a 20 kHz offset to the local oscillatorsignal 37 may be used, followed by finding a nearest integer divisorfrom 20 mHz that will be close to the local oscillator signal 37.Typically, some correction factors are applied such as a gain or anoffset to obtain a signal that accurately represents the rotor position.

In one example, the oscillator signal coming from oscillator 28 is about250 kHz, which is nominal within about 250-254 kHz moving over the rangeof the rotor positioning sensor. In one example, the local oscillatorsignal 37 is preferably set at about 230 kHz. The difference frequencywould be about 20 kHz up to about 24 kHz and a sum of the frequencieswould be about 480 kHz to about 484 kHz. The signal is then directedthrough the low pass filter 32 that filters out the summed frequency sothat the low frequency component remains.

The low frequency component is then directed to the comparator 34. Thecomparator 34 may be a squaring comparator that provides a square waveoutput in the range of about 20 kHz. The signal from the comparator 34may be directed to a timer/comparator input of the microprocessor 36,which may also be referred to as a clock input, counter input or highspeed counter. In at least one example, a time interval measurementbetween when the signal crosses zero becomes proportionate to a positionof the rotor relative to the stator. The microprocessor 36 corrects forcalibration errors and converts the signal to a number, possiblyrepresented by a voltage range that is appropriate for use by the rotorpositioning member (e.g., the voice coil).

By putting the oscillator 28 within a housing of the pump 12, the signaldelivered to the microprocessor in the controller 14 and received backfrom the microprocessor 36 to the rotor positioning member is much lesssensitive to minor voltage changes that may be induced with the cablethat interconnects the pump 12 and controller 14. Some arrangementsinclude positioning the oscillator 28 at other locations, such asadjacent to the controller 14 at a location remote from the housing ofthe pump 12.

Another aspect of the present disclosure relates to theinterchangeability of the pump 12 and controller 14 with other pumps andcontrollers. The calibration constants for the pump may be stored inmemory (e.g., an ID chip) of the pump, and calibration constants for thecontroller may be stored in memory of the controller. For example, thepump may have calibration constants A, B, C that are stored in memory ofthe pump. The controller may have calibration coefficients D and E thatare stored in memory of the controller. When the pump is connected tothe controller, an ID chip of the pump is connected in electroniccommunication with the controller. The pump calibration coefficients aredownloaded to the controller. The controller uses those pumpcoefficients to correct the frequency of the signal received from theoscillator of the pump. For example, the pump parameters are used to setthe local oscillator signal 37 in FIG. 3 so that the correct localoscillator frequency 37 is used for calibrating the frequency that isbeing received from the oscillator 28 of the pump.

In one example, the pump parameters are determined during manufacture ofthe pump. The pump is tested during manufacturing to receive certaindata such as, for example, a frequency measurement from the oscillatorwhen the rotor is moved longitudinally to the inlet and anotherfrequency measurement when the rotor is moved to the outlet of thestator. The various pump parameters may be calculated based on these twofrequency measurements. The pump parameters may be used to determine,for example, the local oscillator divisor or signal, a pump gain, and apump offset. Those parameters are stored in memory of the pump (e.g., onan ID chip of the pump). Similarly, the controller is tested duringmanufacturing to determine a DAC divisor and DAC offset, which isessentially a first order correction to the sensor output 43 of FIG. 3.

In one example, the pump includes about 20 to 25 circuit components aspart of the oscillator 28 that are positioned inside the pump housing.Of these components, 4 to 6 may critically affect the accuracy of theoscillator. The controller may include multiple resistors (e.g.,preferably about 4 to 5 resistors) that set gains and offsets for thecontroller output. The parameters of the pump and controller aretypically relatively stable in the presence of variations intemperature. Further, a crystal 39, which is typically stable withchanges in temperature, is used in connection with the microprocessor 36to even further stabilize the pump system 10.

Another aspect related to the pump system disclosed herein relates tothe number of components and the related complexity of the systemcorresponding to the number of components. Some types of pump systemsinclude at least 50 components associated with the accuracy of theposition sensor circuitry. The numerous amplifier stages and associatedgains and offsets for these components all affect the performance of thepump. The pump of the present disclosure uses only a few criticalcircuit components, which are primarily for the oscillator. The pumpcircuit components may be thermally stabilized using known techniques,such as negative temperature coefficient capacitors and properties ofthe sensor coil and the larger capacitors that make up the oscillator.The pump circuitry may be simpler to manufacture, test and maintainbecause of its few number of critical components.

Another advantage related to the reduced number of components in thepump system 10 as compared to other pump systems relates to the powerusage of the pump system. Other rotor sensor systems use as many as atleast 100 to 150 circuit components for the pump and controller, whichmay consume several watts of power. The rotor position sensor system ofthe present disclosure may be configured to consume power in the orderof 60 to 1,000 milliwatts and more preferably in the range of about 20to 100 milliwatts. In scenarios where the pump system 10 isbattery-operated, the amount of power consumption can be an importantdesign factor. Furthermore, the increased use of power and the number ofcomponents affects the amount of heat generated within the pump housingand controller housing. The reduced number of components in the pumpsystem of the present disclosure and the related decreased amount ofheat generated may lead to improved consistency in performance,increased component life, and increased battery life.

Referring now to FIGS. 5 and 6, several example methods of determining arotor position in a stator housing are described. FIG. 5 shows anexample method 200 that includes a step 202 of providing a statorhousing having a fluid pathway, a rotor hub positioned in the fluidpathway, a sensor coil positioned external to the fluid pathway, aconductive target carried by the rotor hub, and an oscillator. A step204 includes changing coupling of the eddy current target to the sensorcoil by moving the rotor hub longitudinally relative to the statorhousing, thereby altering an inductance of the sensor coil. The sensorcoil operates as a resonating element in the oscillator. A step 206includes determining a position of the rotor hub relative to the statorhousing using an output of the oscillator.

Other steps of method 200 may include shifting a frequency of a signaloutput from an oscillator to create a lower frequency signal, measuringthe lower frequency signal, and providing a value for the measured lowerfrequency signal that represents a position of the rotor hub relative tostator housing. Another step for method 200 may include locking a phaseof the signal output from the oscillator to create a phase lockedsignal, measuring the phase locked signal, and providing a value for themeasured phase locked signal that represents a position of the rotor hubrelative to the stator housing. A further method step may includelocking a frequency of the signal output from the oscillator to create afrequency locked signal, measuring the frequency locked signal, andproviding a value for the measured frequency locked signal thatrepresents a position of the rotor hub relative to the housing. Themethod 200 may also include magnetically-levitating the rotor hub in thefluid pathway, and controlling a longitudinal position of the rotor hubrelative to the stator housing in response to the determined position ofthe rotor hub.

FIG. 6 illustrates a method 300 of determining a position of a rotorwithin a housing in a magnetically-levitating pump system. The method300 includes a step 302 of providing a magnetically-levitating pumpsystem having a pump and a controller. A step 304 includes providing thepump with a rotor positioned in a stator, a position sensor positionedoutside of the stator, and an oscillator, wherein the position sensorcomprises a coil. A step 306 includes creating resonance in theoscillator with the position sensor in response to a change in relativeaxial position of the rotor to the stator. A step 308 includesprocessing an output signal of the oscillator with the controller tocreate a value representative of the change in relative axial position.A step 310 includes correcting a position of the rotor based on thevalue.

The position of the rotor may be corrected using a voice coil. Themagnetically-levitating pump system may further include a magneticbearing and a magnetic motor. The controller may include amicroprocessor configured to determine the rotor position value usingthe output signal of the oscillator. The controller may include amultiplier, a low-pass filter, a comparator and an amplifier to createan output signal having the value.

The term “resonant eddy current sensor” as used herein may generallyrefer to the class of measuring based on currents induced in anon-magnetic or non-contacting surface. The resonant eddy current sensoras disclosed herein may be used for measuring position based on magneticor on electrical currents induced by a magnetic field from a coil thatis positioned somewhere that is not touching the rotor. The term“frequency shifting” as used herein may relate to shifting the frequencyof resonance or the signal that is the resonant frequency down to afrequency where the signal may be more readily measured.

A “phase lock resonant eddy current sensor” may receive the output ofthe oscillator 28 of the pump 12 and direct the output signal to a phasedetector (e.g., delta phase) where the output signal is summed togetherand integrated. This summed and integrated signal is returned back forcomparing to the oscillator output. The voltage (VCO) is adjusted to beanalogous of the frequency and is called a control voltage in a phaselock loop. From there, the voltage may be digitized and used in alevitation system.

A phase locked resonant eddy current sensor may have similarities to afrequency locked resonant eddy current sensor. A phase locked resonanteddy current sensor measures the phase between two signals and controlsthe voltage based on that comparison. A frequency locked resonant eddycurrent sensor measures frequency and adjusts the voltage up and down ona DC arc to control the frequency.

The principles of the coil positioning sensor disclosed herein may beapplied to magnetically levitated motor and magnetically levitatedpumps, and specifically to magnetically levitated blood pumps.Principles disclosed herein may be useful in other applications outsideof blood pumps where accurate measurements of the rotor positionrelative to the stator and obtaining a position signal relatively freeof influence from outside environmental conditions is desired.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the inventionincludes all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims. It is specifically noted that any features or aspectsof a given embodiment described above may be combined with any otherfeatures or aspects of other described embodiments, without limitation.

What is claimed is:
 1. A pump system configured to provide fluid flow,comprising: a stator housing having an inlet and an outlet and a fluidpathway; a rotor disposed within the fluid pathway between the inlet andthe outlet, the rotor hub comprising a body having a leading portionpositioned adjacent the inlet, a trailing portion positioned adjacentthe outlet; an eddy current sensor coil positioned external the fluidpathway and operable to determine a position of the rotor hub relativeto the stator housing, the sensor coil operating as a resonating elementin a low-frequency oscillator.
 2. The pump system of claim 1, furthercomprising a frequency shifting device that shifts a frequency of anoutput signal from the oscillator to a lower frequency signal.
 3. Thepump system of claim 2, further comprising a frequency measurementcircuit that measures the frequency of the lower frequency signal andoutputs a value representative of a position of the rotor hub relativeto the stator housing.
 4. The pump system of claim 3, wherein the valueoutput from the frequency measurement circuit is in the form of a binarynumber, an electrical current, or an electrical voltage.
 5. The pumpsystem of claim 1, further comprising a phase-locked loop that locks aphase of an output from the oscillator.
 6. The pump system of claim 1,further comprising a frequency-locked loop that locks a frequency of anoutput from the oscillator.
 7. The pump system of claim 1, furthercomprising a memory element containing at least one of calibration,characterization and correction parameters for the sensor coil.
 8. Thepump system of claim 2, further comprising a high speed counterconfigured to measure an interval of time between cycles of the lowerfrequency signal.
 9. The pump system of claim 2, further comprising ahigh speed counter configured to measure a number of cycles of the lowerfrequency signal over a specified time interval.
 10. The pump system ofclaim 1, further comprising a pump housing, the stator housing, therotor hub, and the sensor coil are positioned in the pump housing. 11.The pump system of claim 10, further comprising a microprocessorpositioned remote from the pump housing.
 12. The pump system of claim 1,further comprising at least one permanent magnet bearing and a magnetmotor, the magnet motor comprising a motor magnet carried by the rotorhub and a motor coil carried by the stator housing, the at least onepermanent magnet bearing levitating the rotor hub within the statorhousing, and the magnet motor operable to rotate the rotor hub withinthe stator housing.
 13. A sensor assembly for a pump withmagnetically-levitating rotor, the sensor assembly comprising: a sensorcoil positioned on a stator of the pump; a rotor of the pump arrangedwithin the stator and having a conductive surface; a low frequencyoscillator positioned within a housing of the pump; wherein the sensorcoil operates as a resonating element in the low frequency oscillator inresponse to a change in relative position between the rotor and sensorcoil, and an output signal from the low frequency oscillator is used toadjust a position of the rotor relative to the stator.
 14. The sensorassembly of claim 13, wherein an output of the low frequency oscillatoris in the range of about 200 kHz to about 350 kHz.
 15. The sensorassembly of claim 14, wherein an output of the low frequency oscillatoris a sine wave signal.
 16. An active magnetically levitating pump systemconfigured to provide fluid flow, comprising: a stator housing having afluid pathway; a rotor disposed within the fluid pathway; an eddycurrent sensor coil positioned external the fluid pathway and operableto determine a position of the rotor with respect to a defined axis ofthe stator housing, the sensor coil operating as a resonating element ina low-frequency oscillator.
 17. The active magnetically levitating pumpsystem of claim 16, wherein the eddy current sensor coil is operable todetermine a position of the rotor with respect to a longitudinal axis ofthe stator housing.
 18. The active magnetically levitating pump systemof claim 16, wherein the eddy current sensor coil is operable todetermine a position of the rotor with respect to a lateral axis of thestator housing.
 19. The active magnetically levitating pump system ofclaim 16, further comprising a pump housing, the stator housing, rotorand eddy current sensor coil being positioned in the pump housing. 20.The active magnetically levitating pump system of claim 16, wherein thelow-frequency oscillator is positioned in the pump housing.
 21. A methodof determining a rotor position in a stator housing, the methodcomprising: providing a stator housing having a fluid pathway, a rotorhub positioned in the fluid pathway, a sensor coil positioned externalthe fluid pathway, and an oscillator; inducing eddy currents in therotor hub via the magnetic field of the sensor coil, which eddy currentsin turn produce magnetic fields that interact with the magnetic fieldsof the sensor coil as the rotor hub is moved relative to the statorhousing; determining a position of the rotor hub relative to the statorhousing using an output of the oscillator.
 22. The method of claim 21,further comprising shifting a frequency of a signal output from theoscillator to create a lower frequency signal, measuring the lowerfrequency signal, and providing a correction value for the measuredlower frequency signal, the correction value representing a position ofthe rotor hub relative to the stator housing.
 23. The method of claim21, further comprising locking a phase of a signal output from theoscillator to create a phase locked signal, measuring the phase lockedsignal, and providing a correction value for the measured phase lockedsignal, the correction value representing a position of the rotor hubrelative to the stator housing.
 24. The method of claim 21, furthercomprising locking a frequency of a signal output from the oscillator tocreate a frequency locked signal, measuring the frequency locked signal,and providing a correction value for the measured frequency lockedsignal, the correction value representing a position of the rotor hubrelative to the stator housing.
 25. The method of claim 21, furthercomprising magnetically-levitating the rotor hub in the fluid pathway,and controlling a longitudinal position of the rotor hub relative to thestator housing in response to the determined position of the rotor hub.26. A method of determining a position of a rotor within a housing in apump with magnetically-levitating rotor system, the method comprising:providing a controller and a pump, the pump having a rotor, a stator, aposition sensor, and an oscillator, the rotor being positioned insidethe stator, the position sensor being positioned on the stator, and theposition sensor comprising a coil; creating a change in frequency in theoscillator with the position sensor in response to a change in positionof the rotor relative to the stator; processing an output signal of theoscillator with the controller to create a correction valuerepresentative of the change in relative axial position; correcting aposition of the rotor based on the value.
 27. The method of claim 26,further comprising correcting the position of the rotor with a voicecoil.
 28. The method of claim 26, further comprising providing acontroller that is positioned remote from the oscillator, the controllercomprising a microprocessor configured to determine the correction valueusing the output signal of the oscillator.